1. Field of the Invention
The present invention relates to a position control method and a position control system for feed drive equipment, and particularly, to a position control method and a position control system for feed drive equipment provided with a plurality of feed screws for feeding a relatively large movable body.
2. Description of the Related Art
Among feed drive equipment for feeding a relatively large movable body, such as a worktable of a large-scale tooling machine, there are ones of a system in which a plurality of feed screws are disposed in parallel, and the servo motors are driven individually by servo motors. This system allows motors of relatively low capacity to be used.
In feed drive equipment of the above-noted system, a position control is made by distributing an identical position command to a plurality of servo motors, to thereby match rotational positions of a plurality of feed screws with each other.
Therefore, if actual pitches are different between feed screw mechanisms, their feeds have a difference therebetween, which causes a couple of forces acting on a movable body in a twisting direction, and lowers a positioning accuracy of the movable body. Further, in the use of servo motors relatively low of capacity, they have to bear loads corresponding to the couple of forces in addition to a load corresponding to a feed of the movable body itself, and may be overloaded. The difference in pitch between the feed screw mechanisms depends on a temperature difference therebetween, as well, and cannot be covered up by simply improving their tooling precision.
In this regard, it is possible to detect respective feeds of the feed screw mechanisms, to thereby make a position control for matching the feeds. However, in that case, there is a need for the position control to be a full-closed system in which the feed screw mechanisms are individually provided with position scales. In general, the scales are troublesome to be cleaned if they are made of glass, or unable to render their precision of position high if they are of magnetic type.
The present invention is made with the above noted points in view. It is an object of the present invention to provide a position control method and a position control system for drive feed equipment by which, even when an actual pitch difference or temperature difference exists between feed screw mechanisms, it is possible by matching the feeds to keep a positioning accuracy of a movable body and prevent overloading the motors, such that this can be achieved in both of a semi-closed system and a full-closed system, and in the case of a full-closed system, it is unnecessary to employ multiple scales.
To achieve the object, according an aspect of the invention, there is provided a position control method for feed drive equipment in which a plurality of feed drive mechanisms disposed in parallel for feeding a movable body are individually driven by servo motors, the position control method comprising: detecting torque of the servo motors, and correcting position commands of the servo motors in dependence on the detected torque so that the servo motors have matching torque.
To achieve the object, according another aspect of the invention, there is provided a position control system for feed drive equipment in which a plurality of feed drive mechanisms disposed in parallel for feeding a movable body are individually driven by servo motors, the position control system comprising: a controller for detecting torque of the servo motors, and correcting position commands of the servo motors in dependence on the detected torque so that the servo motors have matching torque.